Folia Zool. – 57(3): 313–323 (2008)
Diet of larvae and juvenile perch, Perca fluviatilis performing diel
vertical migrations in a deep reservoir
Michal Kratochvíl1,2, Jiří Peterka1, Jan Kubečka1,2, Josef Matěna1,2, Mojmír Vašek1, Ivana
Vaníčková1,2, Martin Čech1 and Jaromír Seďa1,2
1
Biology Centre of the AS CR, v. v. i., Institute of Hydrobiology, Na Sádkách 7, 370 05 České Budějovice,
Czech Republic; e-mail: Michal.Kratochvil@prf.jcu.cz
2
Faculty of Science, University of South Bohemia, Branišovská 31, 370 05 České Budějovice, Czech
Republic
Received 4 April 2007, Accepted 18 April 2008
A b s t r a c t . Feeding behaviour of two functional groups of 0+ perch Perca fluviatilis
(epilimnetic, staying all 24 hours in epilimnion; hypolimnetic, daily migrating between hypolimnion
and epilimnion) were investigated in the deep canyon-shaped Slapy Reservoir (Czech Republic)
during two 24-h periods in late May and mid June 2002. Densities of most favoured cladocerans
and copepods were generally higher in epilimnetic than in hypolimnetic zones. The two 0+ perch
groups fed predominantly on cyclopoid copepods during the daytime in May. In June, epilimnetic
perch fed on cladocerans (Daphnia sp., Diaphanosoma brachyurum), whereas hypolimnetic perch
preferred calanoid copepod Eudiaptomus gracilis. Throughout darkness, when nearly all perch
occupied upper strata, their gut contents were clearly dominated by cladocerans Daphnia sp.
and Diaphanosoma brachyurum in May and June, respectively. Digestive tract fullness (DTF) of
hypolimnetic perch was 2.0–2.8-times lower than the DTF of epilimnetic perch, and a higher share
of perch with empty digestive tracts was found in the hypolimnion. Maximum DTF occurred in
the epilimnion during the day and/or dusk, whereas at night and dawn progressive evacuation of
guts was recorded and migrants returned with low DTF back to the hypolimnion. Low zooplankton
abundance, unfavourable light and temperature conditions in the hypolimnetic zone are suboptimal
both for prey searching and for overall metabolic processes.
Key words: 0+ fish, Slapy Reservoir, digestive tracts fullness, zooplankton
Introduction
A shift from littoral to pelagic habitat occurs (P o s t & M c Q u e e n 1988, M a t ě n a
1995a, U r h o 1996) during the early life history of both species of perch, the European perch
(Perca fluviatilis L.) and its close relative, the North-American yellow perch (Perca flavescens
(Mitchill)) (P o s t & M c Q u e e n 1988, U r h o 1996). Larvae of both species migrate
from the littoral zone into the pelagic habitat soon after hatching, and stay there for a month
or even longer while they feed predominantly on zooplankton (T h o r p e 1977, K o k e š &
S u k o p 1984, M a t ě n a 1995b). Some juveniles then switch to demersal mode of life and
return back to the littoral zone (C o l e s 1981, P o s t & M c Q u e e n 1988, T r e a s u r e r
1988, W a n g & E c k m a n n 1994, U r h o 1996), or to the benthic zone (L i n 1975). It
has been hypothesized that these shifts are connected with depletion of zooplankton resources
in the pelagic area (T r e a s u r e r 1988, W a n g & E c k m a n n 1994) or with higher
predation vulnerability of pigmented, non-transparent juveniles (fully metamorphosed), that
can be detected by cruising pelagic predators more easily than transparent ichthyoplankton
(K e l s o & W a r d 1977, W h i t e s i d e et al. 1985).
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In lakes, maximum abundances of pelagic 0+ perch have been reported from surface
layers of the water column (C o l e s 1981, W h i t e s i d e et al. 1985, P o s t &
M c Q u e e n 1988, T r e a s u r e r 1988, W a n g & E c k m a n n 1994). Consequently,
a lot of studies have focused on the diet of pelagic 0+ perch living in epi- or metalimnion
of reservoirs or lakes (e.g. W h i t e s i d e et al. 1985, J a c h n e r 1991, F l i k et al. 1997,
M a t ě n a 1998). Some studies have reported 0+ perch communities from greater depths
(C o o p e r et al. 1981, P e r r o n e et al. 1983, K u b e č k a & S l a d 1990), but papers
on diet of hypolimnetic and/or vertically migrating populations of 0+ perch are scarce
(S l a d 1988).
Recently, Č e c h et al. (2005) described the distribution of two sympatrically living 0+
perch groups in the pelagic area of a canyon shaped reservoir. The majority of perch larvae
and juveniles utilized the epilimnion (non-migrating fry), but a portion of the pelagic 0+
perch population moved from warm epilimnetic layers during the night to the cold and dark
hypolimnion during the day.
Thus, the main objective of this study was to extend the findings of Č e c h et al. (2005)
and describe the diet of migrating and non-migrating 0+ perch. This study focused on 1) the
assessment of available planktonic prey in epi- and hypolimnetic habitats; 2) qualitative and
quantitative aspects of food intake of migrating and non-migrating perch; and 3) diel patterns
of zooplankton consumption.
Study Area, Materials and Methods
Slapy Reservoir, located in the Czech Republic (49°49’28’’ N, 14°25’58’’ E) is a steep-sided
meso- to euthrophic, dimictic reservoir covering an area of 1392 ha (length 42 km, mean width
313 m), with a volume of 269x106 m3 and maximum depth of 58 m. The average theoretical
retention time reflects a relatively high annual inflow of only 38.5 days (H r b á č e k &
S t r a š k r a b a 1966). The reservoir was constructed as a part of the Vltava River Cascade
during the period 1949–1954. From a fish fauna and fishery perspective it differs from other
canyon-shaped reservoirs in the Czech Republic due to high percid contributions to the
stock (K u b e č k a 1993). In the lacustrine study site characterized by steep shores with
poorly-developed vegetation zones, depth of the thermocline was well below 4 m during the
sampling (Č e c h et al. 2005).
Age 0+ perch were collected in open water zone of the reservoir during two 24-h surveys
on 29–30 May and 17–18 June 2002. Both May and June investigations were divided into
four time periods – day (8:00–19:00), dusk (20:00–22:30), night (0:00–3:00) and dawn
(4:00–6:00). To locate 0+ fish in the water column acoustic observations were performed
using a scientific echosounder (Simrad EY 500) located on the net-towing research
vessel (for more details see Č e c h et al. 2005). On the basis of fish signals, a conical
ichthyoplankton net (2 m diameter frame; mesh size 1*1.35 mm) with a 10 kg weight and a
styrofoam floater was used for sampling fish larvae and juveniles within upper 16 m of the
water column. The length of the connecting line between the floater and the net frame was
adjusted according to required sampling depth. The net was towed 50 m behind the research
vessel for 5 minutes with an average speed of 3–4 km/h as estimated by Garmin eTrex
Summit GPS. A supporting boat with a commercial echosounder (Eagle Ultra Classic) was
used to ensure the exact towing depth of the net. Several separate vertical tows from the deep
layers were done additionally to ensure that fish from the upper strata did not contaminate
314
the net while it was lifted from lower towing depths to the surface. All fish collected were
immediately preserved in ~10% formaldehyde for later analyses.
Zooplankton was collected only during day (16:00) and night (0:30) periods,
simultaneously with 0+ fish. In both sampling periods 5–7 different depth strata were
sampled. In May, a Van Dorn sampler (volume 5.6 l, height 0.5 m with a 40 μm mesh) was
used to collect zooplankton. Nauplii and rotifers were not included in the counts of June
zooplankton because they were consumed in negligible amounts by 0+ perch at that time, so
zooplankton samples were collected using a closing 140-μm plankton net (diameter 24 cm).
During both sampling periods, samples were immediately preserved in 4% formaldehyde
solution.
Temperature and oxygen vertical profiles were measured using a calibrated YSI 556
MPS probe. In June, light penetration through the water column was measured using LICOR
LI-250 underwater light meter. The data on temperature, oxygen and light distribution have
already been published in Č e c h et al. (2005).
Zooplankton and fish diet analyses
In the laboratory at least 2/3 of each zooplankton sample or 250–300 individuals were counted
and identified to genus or species level. Only zooplankton samples from epilimnetic (0–4 m)
and hypolimnetic (9–15 and 10–16 m in May and June, respectively) zones were subjected
to statistical analyses.
Fish were identified (according to K o b l i c k a y a 1981) and enumerated. Their
standard lengths (SL) and wet weights were measured to the nearest 0.5 mm and 0.1 mg,
respectively. The length from the snout tip to the end of the chorda dorsalis for larvae and
standard length (SL) for juveniles were taken. Prey items from the gut of fish up to 15 mm
SL (no stomach differentiated) and from both stomach and gut (fish above 15 mm SL), were
identified to the relevant taxonomic level, counted, and whenever possible, measured from
the top of the head to the base of the tailspine (cladocerans), or to the base of the caudal
rami (copepods). In Leptodora kindtii (Focke), length of the tailspines was used and the total
body size was estimated from the regression between tailspine length and body length after
H o r n i g & B e n n d o r f (1985). Wet body mass of zooplankton was estimated from the
length-volume regression given by H o e h n et al. (1998). Prey volume calculated from
median body length of prey type was converted to wet weight assuming a specific gravity
1.0 g/ml. Digestive tract fullness, DTF (mg wet weight of food per 100 mg wet body weight
of perch) was determined after H y s l o p (1980):
n
DTF= 100
∑ G * (W)
i
-1
i =1
where Gi is the wet weight (mg) of relevant prey type i in the digestive tract and W the wet
body mass (mg) of fish before dissection. In total 575 digestive tracts of perch (size 9–24 mm
SL) were analysed.
For graphical presentation of the stomach content data, A m u n d s e n et al. (1996)
modification of Costello’s method was used. This method relates the frequency of occurrence
(Fi – the share of digestive tracts in which prey i occurs from all filled digestive tracts) to
prey-specific abundance (Pi – percentage a prey i comprises of all prey items in only those
predators in which prey i occurs), and enabled us to determine prey importance and also
315
feeding strategy of predators. Prey taxa close to 1% occurrence and 1% abundance are
negligible in the diet; and conversely prey species approaching the upper right corner of
the diagram (100% occurrence and 100% abundance) are considered as the most important
prey. Points close to 1% occurrence and 100% abundance are considered as a specialization
on certain prey taxa by a few predators; points close to 100% occurrence and 1% abundance
indicate generalized diet of most predators.
Statistical analyses were performed using a t-test to compare the DTF of perch between
the epilimnetic (0–4 m) and hypolimnetic (9–16 m) zones during the daytime. To compare
DTF at different times, one-way ANOVA was applied with day, dusk, night and dawn as
treatment factors. Data on zooplankton densities were analysed using two-way ANOVA with
habitat (epilimnetic, hypolimnetic), time (day, night), or month (May, June) as treatments.
Prior to analysis, the transformation log (x+1) on data was applied, when necessary.
DAY
NIGHT
Zooplankton density (ind./l)
Zooplankton density (ind./l)
0 20 40 60 80 100120140 0 100 200 300 400 500 600 0 20 40 60 80 1001201400 100 200 300 400 500 600
MAY
MAY
0-2
Depth (m)
2-4
5-6
7-8
not sampled
not sampled
9-10
11-13
14-15
Depth (m)
JUNE
JUNE
0-2
2-4
4-7
7-10
10-13
13-16
Cladocera
Copepoda
nauplii
Rotatoria
Fig. 1. Day and night densities (ind./l) of main zooplankton taxa on the vertical profile of Slapy Reservoir in May
and June.
Results
Zooplankton distribution
Densities of cladocerans and copepods were higher in epilimnion than in hypolimnion zones
during daylight periods at both sampling dates (two-way ANOVA; habitat: F1,14, P < 10-6) (Fig.
1). The same densities of cladocerans and copepods in epilimnion zone occurred during day
and night periods (F1,12, P = 0.56) as well as between months (F1,12, P = 0.39).
In May, the cladoceran assemblage was dominated by Daphnia sp. (nearly exclusively
Daphnia galeata Sars) in the epilimnetic zone during both day and night (Table 1).
Bosminidae (particularly Bosmina longirostris (O. F. Müller)) dominated the hypolimnion
zone at night, but Daphnia sp. was the most abundant cladoceran in the hypolimnion
zone during daylight. In June, the epilimnetic zone was dominated by the typical summer
species Diaphanosoma brachyurum (Lievin), and Bosminidae prevailed the hypolimnetic
316
Table 1. Day and night densities (ind./l) of different zooplankton taxa in 0–4 and 9–16 m depth layers in May and
June. Category other cladocera represents Ceriodaphnia sp. and Chydoridae.
May day night 0–4 m 9–15 m 0–4 m 9–15 m
June
day 0–4 m 10–16 m
night
0–4 m 10–16 m
Cladocera:
Bosminidae
Diaphanosoma brachyurum
Daphina sp.
Leptodora kindtii
other
2.6
0.5
97.2
2.2
0.0
10.0
0.0
13.5
0.5
0.8
4.2
1.5
58.8
0.5
0.6
56.9
2.3
28.4
0.0
0.0
1.7
37.6
24.6
0.2
0.0
2.4
0.7
1.2
0.0
0.0
1.8
51.8
37.6
1.8
0.0
10.7
0.3
1.3
0.0
0.2
Copedoda:
Acanthocyclops trajani Cyclops vicinus
Eudiaptomus gracilis
Mesocyclops leuckarti
Thermocyclops crassus
2.1
20.7
30.3
13.8
9.9
3.3
7.7
2.2
0.0
18.4
0.0
48.5
21.2
4.7
5.0
1.8
7.8
1.8
0.6
1.2
14.6
8.3
38.9
78.6
14.2
0.1
0.1
0.6
0.1
0.1
10.0
4.4
32.9
31.4
15.1
0.2
0.1
0.6
0.4
0.1
cladoceran assemblage. Copepodite stages and adult copepods contributed significantly to
the total zooplankton abundances only in the epilimnetic zone (Fig. 1). The detailed species
composition of copepods is also given (Table 1). No evidence for apparent diel vertical
migrations of zooplankton was found during May and June.
Diet of 0+ perch – spatial and diel variability
7.0
6.0
5.0
4.0
3.0
29
30
20
71
14
2.0
73
28
1.0
0.0
DTF (mg wet weight/100 mg wet fish weight)
MAY
)
DAY
DUSK NIGHT DAWN
Time period
DTF (mg wet weight/100 mg we
DTF (mg wet weight/100 mg we
DTF (mg wet weight/100 mg wet fish weight)
Analyses of digestive tract fullness (DTF) revealed different diel patterns in feeding
activity on both investigated dates (Fig. 2). In May, the most intensive feeding was reported
throughout the day (in the epilimnion zone) and during dusk, when the shift of hypolimnetic
JUNE
7.0
44
6.0
5.0
18
4.0
3.0
38
45
2.0
1.0
0.0
88
36
41
DAY
DUSK NIGHT DAWN
Time period
Fig. 2. Mean digestive tract fullness (DTF; ± 1 S.E.) of perch larvae and juveniles during a diel cycle in May and
June. Open circles represent perch in epilimnion, solid dots represent perch in hypolimnion and solid triangles
represent hypolimnetic perch during migrations (5-9 m) at dusk and dawn. Numbers of analysed fish are shown
close to symbols or error bars.
317
EPIPELAGIAL
0-4 m
0
20
40
60
80
100
0
20
40
60
80
100
0.2
0.4
Fi
0.6
0.0
0.2
0.4
Fi
0.6
11.3 ± 2.1 (52)
0.0
12.5 ± 1.8 (28)
DAY
0.8
0.8
1.0
1.0
0.4
Fi
0.6
0.8
1.0
Fi
0.6
0.8
1.0
Daphnia sp.
Fi
0.6
0.8
1.0
40
60
80
100
0
010
Brachionus sp.
Bosminidae
Cyclopidae cop.3-ad.
nauplius
0.0
0.0
0.4
Fi
0.2
0.4
Fi
11.0 ± 1.4 (6)
0.2
0.6
0.6
13.6 ± 2.4 (18)
DAWN
0.8
0.8
Keratella sp.
Leptodora kindtii
Eudiaptomus gracilis
Cyclopidae cop.1-2
Diaphanosoma brachyurum
0
0.4
0.4
0
0.2
0.2
20
40
60
80
100
20
0.0
12.3 ± 2.6 (27)
0.0
13.3 ± 2.8 (58)
NIGHT
20
40
60
80
100
0
0.2
20
40
60
80
100
0
0.0
12.4 ± 2.1 (20)
20
40
60
80
100
DUSK
Pi (%)
1.0
1.0
Fig. 3. Spatial and temporal prey occurrence (Fi) and prey-specific abundance (Pi) in the digestive tracts of perch larvae and juveniles in May, based on prey numbers. Arrows
indicate the dusk and dawn transfers towards and from the surface, respectively. Mean standard lengths (SL) ± 1 S.D. (mm) of analysed fish are given. Numbers of digestive
tracts with any prey content are shown in parentheses.
BATHYPELAGIAL DURING MIGRATIONS
9-16 m
5-9 m
Pi (%)
Pi (%)
Pi (%)
Pi (%)
Pi (%)
Pi (%)
318
319
EPIPELAGIAL
0-4 m
0.2
0.4
Fi
0.6
0
0.4
Fi
0.6
0.8
1.0
0.2
0.4 0.6
Fi
0.8
1.0
0.4
Fi
0.6
0.8
1.0
2
0
Daphnia sp.
010
Brachionus sp.
Bosminidae
0.0
0.0
0.2
0.2
0.4
0.4
Fi
Fi
0.6
0.6
14.9 ± 4.2 (22)
DAWN
Keratella sp.
Leptodora kindtii
Eudiaptomus gracilis
Cyclopidae cop.1-2
Diaphanosoma brachyurum
0
0.2
20
40
60
80
100
0
0
0.0
14.4 ± 3.5 (17)
0.0 0.2 0.4 0.6 0.8 1.0
Fi
20
40
60
80
100
20
40
60
80
100
0
0.0
0
14.0 ± 4.3 (38)
Cyclopidae cop.3-ad.
20
0.2
1.0
20
40
60
80
NIGHT
40
0.0
0.8
12.9 ± 2.9 (44)
20
40
60
80
100
nauplius
12.2 ± 2.1 (80)
0.0
12.4 ± 1.9 (43)
DUSK
60
80
100
0
20
40
60
80
100
0.8
0.8
1.0
1.0
Fig. 4. Spatial and temporal prey occurrence (Fi) and prey-specific abundance (Pi) in the digestive tracts of perch larvae and juveniles in June, based on prey numbers. Arrows
indicate the dusk and dawn transfers towards and from the surface, respectively. Mean standard lengths (SL) ± 1 S.D. (mm) of analysed fish are given. Numbers of digestive
tracts with any prey content are shown in parentheses.
BATHYPELAGIAL
9-16 m
DAY
Pi (%)
100
Pi (%)
DURING
MIGRATIONS
5-9 m
Pi (%)
Pi (%)
Pi (%)
Pi (%)
Pi (%)
perch towards surface occurred. During night, DTF decreased and was the lowest at dawn
(ANOVA, F3,144, P = 0.004). In June, DTF of perch clearly peaked during dusk and the lowest
DTF was found at dawn again (ANOVA, F3,181, P < 10-6). As in May, migrating individuals
returned back to the hypolimnion zone with relatively empty digestive tracts. During
daylight, DTF was significantly lower in hypolimnetic perch during both May and June
(t-test, d.f. = 100, P < 0.001 for May and t-test, d.f. = 143, P < 0.001 for June, respectively)
(Fig. 2). Moreover, a higher ratio of individuals with empty digestive tracts was reported in
hypolimnetic perch (28.8% and 9.1% in May and June, respectively) than in non-migrants
(3.4% and 4.4% in May and June, respectively).
Pelagic 0+ perch fed almost exclusively on planktonic microcrustaceans. In May, younger
(C 1-2), and older (C 3-5) copepodite stages and adult cyclopoid copepods were favoured by
nearly all epilimnetic perch during daylight, whereas hypolimnetic perch were more focused
on older copepodite stages and adult cyclopoid copepods (Fig. 3). During dusk nearly all
of the prey taxa contributed equally on average by about 20% to the diet of the epilimnetic
and migrating hypolimnetic perch, but their occurrence in digestive tracts was of different
importance. Daphnia sp. was the most important prey category found in the digestive tracts
of epilimnion perch during the night and at dawn. In perch migrating to the hypolimnion
zone, the most important prey taxa were again older copepodite stages and adult cyclopoid
copepods, and the importance of Daphnia sp. in the digestive tracts decreased rapidly. In
June, the highest occurrence was reported for Diaphanosoma brachyurum in the digestive
tracts of most (> 90%) perch individuals in the epilimnion zone throughout the diel cycle
(Fig. 4). This species’ contribution ranged between 50–70% with the highest values recorded
at dawn. The diet of most migrants returning back to hypolimnion was constituted mainly by
D. brachyurum. During daylight, Eudiaptomus gracilis (Sars) was the most common prey
in the majority of hypolimnetic perch. This prey still remained the most important in the
digestive tracts of hypolimnetic perch migrating upwards. Other prey taxa did not exceed
30% of the relative abundance at dusk.
Discussion
This study has revealed some notable differences in food intake of two functional groups
of 0+ perch. The main differences were found in the digestive tract fullness (DTF) between
epilimnetic and hypolimnetic perch in both months. The lower amount of prey and higher
percentage of fish with empty digestive tract suggest lower feeding activity of hypolimnetic
perch during both sampling occasions. This lower feeding activity of perch in the hypolimnetic
habitat could be caused by three factors.
First, the densities of the most favoured prey (cladocerans and copepods) were much
lower in the hypolimnetic zones and reached only 21% and even 2.4% of the epilimnetic
densities in May and June, respectively. Similar densities of cladocerans and copepods were
recorded in the epilimnion zone through day and night periods and no DVMs of zooplankton
occurred (pers. obs.). Thus migrations of 0+ perch do not seem to follow any potential prey.
Second, perch are considered as visually oriented particulate feeders adapted to high
light conditions (A l i et al. 1977); therefore it is unlikely that hypolimnetic light conditions
would allow them to feed efficiently. However even in the hypolimnetic dark layers, with
the light intensity changing from 0 to 60 lx (less than 0.05% of the surface light intensity;
Č e c h et al. 2005), perch are able to detect prey, but their reaction distance is considerably
320
restricted (R i c h m o n d et al. 2004). Similarly, diel-feeding activity of the epilimnetic
perch coincided with light intensity. Highest DTF was reported throughout the day and/or
dusk, DTF decreased throughout the night and DTF was lowest at dawn, when migrating
individuals returned to hypolimnetic zone. Such a diel cycle of feeding activity corresponds
with results of P o s t & M c Q u e e n (1988) and M a c h á č e k & M a t ě n a (1997).
Besides prey density and light conditions, temperature should have a significant effect on
feeding and survival of hypolimnetic perch. Optimal temperatures for perch larvae were
found at 16–26°C (W a n g & E c k m a n n 1994). Results of K u d r i n s k a y a (1970)
showed that 80–90% of perch larvae stopped feeding and decreased their growth rate up to
five times at temperatures <16°C. In Slapy Reservoir, temperatures in the hypolimnion were
well below 12°C (Č e c h et al. 2005), and so the hypolimnetic perch larvae and juveniles
stayed in unfavourable conditions, which could significantly reduce their metabolic activity
(B r e t t 1971) and growth (M o o i j et al. 1994). Upward migrations to warmer layers,
on the other hand, would stimulate digestion (W u r t s b a u g h & N e v e r m a n 1988),
and return of perch to epilimnion zones (16–23°C) during dusk seems to be connected with
increased feeding rate. However, staying in cold hypolimnion during the daytime and feeding
in warm epilimnion overnight appears unlikely to be an energetic advantage as mentioned by
G l i w i c z & J a c h n e r (1992) on juvenile smelt.
Comparison of diet composition in larval and juvenile perch revealed slight differences
between perch in epi- and hypolimnion. Epilimnetic perch preferred cyclopoid copepods
in May and focused on cladocerans in June. Hypolimnetic perch consumed predominantly
copepodites (cyclopoids in May and the most abundant copepod E. gracilis in June) and
did not prefer cladocerans, although cladocerans generally dominated in the hypolimnion
(except the situation from the daytime in May). Feeding on small, evasive copepods
(J a c h n e r 1991, F l i k et al. 1997, E a s t o n & G o p h e n 2003) seems to be less
profitable, and can even diminish growth (R o m a r e 2000). The preference for copepods
was unexpected especially under low light conditions in hypolimnion when larger and more
easily detectable daphnids would seem to be the more appropriate prey (M i l l s et al. 1986,
F l i k et al. 1997). In our case, 0+ perch switched to large and more visible Daphnia sp.
during low light conditions in May, and this prey became dominant in digestive tracts at
night. An increase of preference for D. galeata at dusk and night was also mentioned in
M a c h á č e k & M a t ě n a (1997). In June, however, the majority of perch focused on
the epilimnetic dominant cladoceran species – D. brachyurum throughout the whole 24-h
period.
The main question, why a part of 0+ perch perform migrations to hypolimnion, remains
still unanswered. Hypolimnetic individuals were found to decrease or even stop their
feeding activity in the hypolimnetic zones, most probably due to low zooplankton densities,
insufficient light intensity and cold water. Although no foraging benefit of the hypolimnetic
strategy was found, it is supposed that more important stimulus to trigger these shifts
could be a predator fear, most probably caused by in Slapy reservoir dominant large perch
(D r a š t í k et al. 2004). A part of 0+ perch is assumed to migrate downward to avoid
potential predators (fish > 100 mm), which are very scarce in hypolimnion of deep, stratified
reservoir (Č e c h & K u b e č k a 2006). The epilimnetic 0+ perch group choose a strategy
of higher predatory risk, but sufficient feeding all the time. Therefore it is most likely, that
the group living in food-rich epilimnion grows faster, but suffers a higher mortality. The
opposite trend is expected for hypolimnetic migrants. However, further and more detailed
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investigations should be performed to evaluate the nutritional conditions of both segregated
0+ perch groups using the RNA: DNA ratio (C l a p p & D e t t m e r s 2004), estimate
predator pressure in habitats above and below the thermocline, and to reveal ultimate
consequences of the two early life strategies.
Acknowledgements
We are much obliged to Z. P r a c h a ř , M. M a c h á č e k , P. Š t a f a , J. F r o u z o v á , M. P r c h a l o v á
and V. D r a š t í k for their help in the field. We are most grateful to Donald C. J a c k s o n for his helpful
comments on an earlier version of the manuscript and English revision. The study was supported by the projects
No. IAA60017502, AV0Z 60170517 and 1QS 600170504 of the Grant Agency of the Czech Academy of Sciences
and No. 206/06/1371 of the Grant Agency of the Czech Republic.
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